I haven't used the Blast but I have used the Exo and Zero, which I'm pretty sure have the same straps. I've used them in hot conditions (up to 38 degrees C) and haven't really noticed a problem. Sure you sweat - I do without a pack at that temperature - and I noticed sweat stains around the stitching of the daisy chain on the front, but I don't think there was any significant difference from packs I've used with mesh on the shoulder straps.
Personally, I think if you're wearing a top that wicks well, it has a much greater affect than shoulder strap mesh. If, however, you hike topless, you MAY find the Blasts unbreathable straps to be less comfortable on bare skin. I'm speculating, I haven't tried it. I do, though, exercise on the steps at home with a pack with mesh on the straps and they get really wet with sweat when used on bare skin.